X-rays are a penetrating electromagnetic radiation typically generated by accelerating electrons to an elevated velocity and suddenly stopping those electrons by means of collision with a solid body. X-rays may also be generated by inducing innershell electron transitions in atoms having atomic numbers greater than about 10. X-rays are typically possessed of wave lengths from about 0.06 to about 120 angstroms and may also be know as roentgen rays.
X-rays have found substantial utility in providing pictures of objects otherwise normally concealed from sight to the human eye. Most particularly, X-rays have found great utility in the medical industry where, because of differences in the relative opacity of various portions of internal organs and structures of the human body, the projection of X-rays through the body onto an electromagnetic sensitive film can produce a representation of the shape and form of the structures within the body. Depending upon the angular positioning of an X-ray generator and the film with respect to the body, and upon making repeated film exposures at a plurality of such angles a 3-dimensional view of these body structures can be achieved. Computers find utility in enhancing such views.
X-ray devices employed particularly in the medical field generally utilize X-rays generated by a vacuum tube or so-called X-ray tube configured to produce X-rays by accelerating electrons to an elevated velocity by means of an electrostatic field and then suddenly stopping those electrons by collision with an interposed target. The operation of such an X-ray tube generates a significant quantity of heat, the dissipation of which is hindered by the inherently non heat conducting nature of a vacuum tube. Where, as in early medical X-ray devices, only single so-called shots or photographic X-ray images were taken at times somewhat separated one from the next, the accumulation of heat generated by the operation of an X-ray tube did not substantially interfere with the routine operation and use of such X-ray machines. More recently, however, X-ray medical devices have been developed wherein it is desired that a considerable number of X-ray photographs be taken at varying angles with respect to the body in a relatively short period of time. In these devices, such as so-called cat-scanners, one limitation upon the rapidity with which X-ray photographic images may be obtained from the cat-scanner is the dissipation of heat that builds up within the X-ray tube during generation of X-rays for producing such X-ray cat-scan photographs.
One factor contributing to the relatively slow dissipation of heat from X-ray tubes is the basic configuration of the tube. Typically, such tubes are formed as a glass or glass-like envelopes of generally cylindrical configuration, the interior of which is normally evacuated to a vacuum of between 10.sup.-6 and 10.sup.-7 torr.
Within the envelope a cathode typically is positioned in electrical communication with a source of relatively elevated electrical potential position generally external to the envelope. Also located within the envelope is typically a so-called target and track assembly generally formed as a disk oriented approximately perpendicularly to a longitudinal axis of the envelope. The disk like target includes generally a track adhered to the target typically adjacent an outward circumferential edge and oriented in a direction generally facing the cathode. As a result of the track being offset from a central axis of the envelope, the cathode is generally oriented at a position within the envelope facing the track but offset from a longitudinal axis of the envelope.
The target and track assembly is typically supported within the envelope employing a shaft which protrudes through the envelope to a connection with the source of electrical potential and to a drive means for rotating the shaft and thereby rotating the target and track assembly within the envelope. Where the shaft passes through the envelope, the shaft is typically supported and spaced from the envelope by bearings; these bearings typically also function to maintain a vacuum within the envelope. Such bearings generally have a service temperature limitation of between approximately 200.degree.-300.degree. C.
The envelope including cathode and target track assembly typically is contained within a canister including dielectric oil at least partially filling the canister. The canister includes a beryllium "window" through which X-radiation may exit the envelope and surrounding canister for use in performing X-ray functions.
Typically, heat arising from the electromagnetic generation of X-rays accumulates in the target of the X-ray tube. Heat may be eliminated from the target by either radiation through the vacuum tube and into the dielectric oil or by thermal conductance along the shaft to a point external to the vacuum envelope. As the shaft typically is of relatively elongated axial length relative to its cross-sectional area, conductance along the shaft has not generally proved to be an effective and efficient means for removing heat from the cathode ray tube. Further, should the shaft become heated to a point exceeding about 200.degree. C. in dissipating heat acquired from the target, the bearing supporting the shaft at the shaft passage through the envelope could suffer deleterious consequences. Likewise, radiation of heat from the target track assembly of the X-ray tube has proven less than satisfactory in dissipating the heat evolved by the generation of X-rays. One factor contributing to less than satisfactory by heat dissipation radiation has been the relatively small surface area available at the target for radiation of heat. Further, such target areas are generally formed of a metal alloy such as so-called TZM alloy, an alloy of titanium, zirconium and molybdenum. Such metal alloys typically have a relatively low surface emissivity constant which typically exerts a depressing effect upon the quantity of heat which can be rejected from the target by radiation per unit of time.
It has been suggested that a fine grain carbon be applied in laminate manner to the TZM target to provide a larger heat reservoir and an expanded surface area for radiation of heat. Such proposals, however, have not satisfactorily addressed the ultimate difficulty in providing an X-ray tube capable of a substantial throughput, that is a relatively large number of X-ray discharges from the tube during a relatively brief period of time. Such a capacity requires a large step change rather than an incremental change in the capability for the X-ray tube to reject heat evolved during the generation of X-rays. Small step changes in the capability for the X-ray tube to accumulate heat and provide for its rejection do not satisfactorily provide for large increases in the number of X-ray discharges required from the X-ray tube per unit of time. Were an X-ray tube available having the capability for rejecting relatively large quantities of heat per unit of time, such tubes employed in the generation of X-rays for industries and science could substantially boost productivity where X-rays are used for the performance of necessary tasks in these industries.